A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is known to clamp the patterning device onto a support during a lithographic process. With this the support includes a clamping mechanism. For example the patterning device may be clamped with membranes in order to constrain X, Y and Rz positions of the patterning device, that is to say in order to constrain its planar movements. In addition, on the lower side of the patterning device three support points may be provided in combination with an air cushion in order to determine the Rx, Ry and Z position. In this way, the patterning device will only deform by the stiffness ratio between the membranes and the patterning device which is about 500 ppm.
However, due to the friction-based clamping of the patterning device, in combination with finite stiffness in the clamping mechanism, micro-slip and hysteresis occur in the position of the patterning device. The hysteresis depends among others on acceleration of the support during the lithographic process, vacuum forces of the clamping mechanism, contamination of the patterning device, humidity, and choice of material for the patterning device. Part of the hysteresis is macroscopic and reproducible for the up and down motions of the support, that is to say in the Z-direction. A significant portion, however, is microscopic, that is to say within the patterning device, and does not reproduce. This causes internal stress in the patterning device and a significant contribution on overlay, which will increase to tens or even hundreds of nm for this type of clamping mechanism.
U.S. Pat. No. 6,094,255 discloses a variant embodiment in which the patterning device is kept floating at a desired distance above a floating plate of a support. The three support points at the lower side of the patterning device are then dispensed with. Instead, a controller is provided which is designed to adjust the pressure of air blown from air holes below the patterning device in such a way that the patterning device is floated. The position of the patterning device in its x-y plane is controlled in accordance with measurements taken by a position measuring apparatus and is controlled by selectively pressing the respective side faces of the patterning device. For exerting these pressing forces, a large number of control bars is in contact with the side faces of the patterning device. The controller also controls the driving of linear motors having stators and sliders, thereby adjusting the pressing forces of the control bars.
The floating or levitation of the patterning device has the benefit that it does not contact any surface of the support. Deformation of the patterning device on the one hand is thus minimized and on the other hand is somewhat more reproducible in order to be able to minimize overlay errors.
Nevertheless, the construction with the linear motors and control bars is somewhat expensive and, for example, the overlay performances with this type of patterning device support may not be sufficient.